1. Field of the Invention
This invention relates to an improved process for producing an aliphatic polyimide resin from an aliphatic tetracarboxylic acid and a diamine.
2. Description of the Prior Art
In general, polyimides are produced by reacting a tetracarboxylic dianhydride or a derivative thereof with diamines or diisocyanates. As is well known in the art, polyimides thus-produced have excellent thermal, electrical and mechanical properties, but they are generally not sufficiently satisfactory in flexibility, moldability, solubility, and other properties.
Aromatic polyimides are particularly subject to the disadvantages as described above. In commercially producing the aromatic polyimide resin, a solution of a polyamide acid, i.e., a polyimide precursor, is prepared, and after molding, the polyamide acid is converted into an imide by heating or using a cyclization-dehydrating agent such as acetic anhydride, since the aromatic polyimide resin itself is insoluble in solvents and infusible. In this case, therefore, a large amount of expensive polar organic solvent, e.g., dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone or the like is required, and the solution is inferior in stability because of ring closure, moisture absorption, etc., which occur during the storage thereof.
The aromatic polyimide resins, therefore, are used only in limited industrial applications irrespective of their excellent characteristics; that is, they have been used in particular situations where their particular high qualities are required.
On the other hand, those polyimides obtained by using aliphatic tetracarboxylic acids as an acid component have significantly higher solubilities in solvents. Therefore they have good workability in the state of solution, and inexpesive solvents can be used in preparing such solutions. Thus it is possible to use such aliphatic polyimide resins in a wide variety of industrial applications such as an electrical insulative material, a varnish, an adhesive, a film, and so forth.
In the production of these aliphatic polyimide resins, various problems occur because of the strong interaction between the aliphatic carboxyl group and amine group. For example, the groups form rigid salts, sometimes preventing the reaction from proceeding uniformly, and in proceeding sufficiently the reaction to obtain polymeric products, cross-linking and gelation may easily occur during the reaction. This reduces the solubility of the polyimide resins in organic solvents, and adversely affects the properties of the polyimide resins, particularly flexibility, moldability, etc. Moreover, in attempting to obtain high polymeric products by allowing the reaction to proceed sufficiently, the desired product may not be obtained.